Vol. 45 No. 5
SCIENCE IN CHINA (Series C)
October 2002
Negative phototropism of rice root and its influencing
factors
WANG Zhong ( ), MO Yiwei (), QIAN Shanqin ()
& GU Yunjie (
)
Agronomy Department of Agriculture College, Yangzhou University, Yangzhou 225009, China
Correspondence should be addressed to Wang Zhong (email: [email protected])
Received July 26, 2002
Abstract Some characteristics of the rice (Oryza sativa L.) root were found in the experiment of
unilaterally irradiating the roots which were planted in water: () All the seminal roots, adventitious
roots and their branched roots bent away from light, and their curvatures ranged from 25° to 60°.
The curvature of adventitious root of the higher node was often larger than that of the lower node,
and even larger than that of the seminal root. () The negative phototropic bending of the rice root
was mainly due to the larger growth increment of root-tip cells of the irradiated side compared with
that of the shaded side. () Root cap was the site of light perception. If root cap was shaded while
the root was irradiated the root showed no negative phototropism, and the root lost the characteristic of negative phototropism when root cap was divested. Rice root could resume the characteristic of negative phototropism when the new root cap grew up, if the original cells of root cap were
well protected while root cap was divested. () The growth increment and curvature of rice root
were both influenced by light intensity. Within the range of 0100 µmolm−2s−1, the increasing of
light intensity resulted in the decreasing of the growth increment and the increasing of the curvature of rice root. () The growth increment and the curvature reached the maximum at 30 with
the temperature treatment of 1040. () Blue-violet light could prominently induce the negative
phototropism of rice root, while red light had no such effect. (
) The auxin (IAA) in the solution, as
a very prominent influencing factor, inhibited the growth, the negative phototropism and the gravitropism of rice root when the concentration of IAA increased. The response of negative phototropism of rice root disappeared when the concentration of IAA was above 10 mg L−1.
Keywords: rice, seminal root, adventitious root, negative phototropism, gravitropism.
Light, as the most extensive and distinctive environmental signal, affects all aspects of plant
development, including metabolism, organ initiation, morphogenesis and tropic movement, etc. A
complex network of photoreceptors and signaling pathway has evolved to regulate development
responses to light intensity, quality, irradiance direction and photoperiod[1
ü3]
. For example, the
plant stem will generate tropic movement to obtain light energy for photosynthesis, if the two
sides of the stem get different irradiance.
People usually realize that plant stems possess the characteristics of positive phototropism
and negative gravitropism, while roots only possess the characteristics of gravitropism[3,5]. Although there have been a few reports on root negative phototropism since one century ago[4], the
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existence of significant root negative phototropism was not widely appreciated by scientists until
Okada’s experiments on Arabidopsis in the 1990s[5, 6]. Okada et al. found that the roots of wild
type of Arabidopsis bent 45° away from light, while starchless mutants of Arabidopsis bent directly away from light[5], when the Arabidopsis cultured in the crystal agar media was unilaterally
irradiated. Then people gradually realized that there was a complex relation between the negative
phototropism and the gravitropism of root, the gravitropic growth of root was not only affected by
gravity, but also stimulated by light to a large extent, and the growth curvature of the negative
phototropism was the vector sum of their interactions[6, 7].
In recent years, we found that all the seminal roots, adventitious roots and their branched
roots of rice (Oryza sativa L.) possess distinctive characteristic of negative phototropism[8]. Furthermore, we found that the roots of plants, such as Echinochloa crusgalli L., Leptochloa chinensis L., Eleusine inclica L., Setaria viridis L. and Eclipta prostrata L., also possess the characteristics of negative phototropism. We further studied the perception site of negative phototropism and
its influencing factors.
1
Materials and methods
1.1
Materials for experiment
Two rice (Oryza sativa L.) varieties “Sanlicun” (large panicle variety) and “Liangyoupeijiu”
(hybrid rice variety) were used as experimental materials. The former one was used to study the
negative phototropism of the seminal roots, and the latter one was for the study of the adventitious
roots.
1.2 Study of the growth of rice root
1.2.1
Cultivating and growth study of the seminal roots.
The rice seeds of “Sanlicun” were
soaked in warm water at 30 for 2 d, and germinated on damp gauze. Orderly germinating seedlings were chosen for experiments. Pieces of metal filaments were cut approximately 3 cm in
length. One end of the metal filament was tied to a germinating seed and the other was inserted
into a foamed plastic mass, which floated on the water surface. The germinating seminal roots and
part of the seed were soaked in the water. The foamed plastic mass was a little smaller than the
glass tank, so it could float freely on the water surface, and the growth status of rice could be easily observed because it was close to the side of the glass tank (fig. 1, left). The foamed plastic
mass could be easily taken out of the water for some treatments on the rice roots, such as turning
rice seeds, divesting root cap or cutting part of root tip. The response of gravitropism could be
studied by turning the seeds horizontally.
1.2.2 Cultivating of the adventitious root.
The seeds of “Sanlicun” were sown by stages in
water or in field, and adventitious roots of different leaf-age seedling were cultivated in the glass
tank. Take the example of breeding adventitious roots of tillering node (fig. 1, right). The tillering
seedlings were taken, their tillers were keeled and roots were cut off. Then the main caudexes
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were inserted into some round holes (1 cm in diameter) wad with plastic sponges in the foamed
plastic mass, which was floating on water or the culture solution in a glass tank. The main caudexes were vertically planted 2 cm under water surface. The cultivating of adventitious roots of
elongation node was similar to that of the tillering node.
Fig. 1. Device for study of the growth and negative phototropism of rice roots. Left, Cultivating of the seminal roots;
right, cultivating of the adventitious roots.
1.2.3 The treatments.
The negative phototropic bending of rice root could easily be induced
by unilateral irradiance, and directly observed and photoed through the tank side.
Focus lamps of 2040 W were chosen as the light source. Light intensity was regulated by
adjusting the distance between the light source and rice roots. Light quality was regulated by inserting light filters between light source and rice roots. Eleven interferometric filters were chosen
as the monochromatic components with absorption peaks respectively at 409 nm, 434 nm, 478 nm,
504 nm, 538 nm, 584 nm, 607 nm, 635 nm, 668 nm, 703 nm, and 713 nm. Each absorption was
1020 nm wide. Covering the glass tank with a paper box could control the photoperiods and
dark periods. Sticking black bands on the tank side or wrapping foil around different parts of the
rice root could shade the roots partially.
The temperature was controlled by moving the rice root together with the glass tank into a
breeding box. The effect of temperature on the negative phototropism could be observed when the
roots were irradiated through the transparent glass.
Effects of different chemical reagents on the negative phototropism of rice roots were studied
by changing the ingredients of the solution in the glass tank. Chemical reagents used in this
experiment included auxin (IAA), gibberellin (GA), and colchicine, etc.
1.2.4 Measurement of negative phototropism.
Growth statuses of rice roots under different
treatments were photoed by digital camera, and the image information was inputted into a computer. The growth increments in unit time were measured, the growing rates of rice roots were
calculated, and the curvatures of the negative phototropism and gravitropism of rice roots were
measured with a protractor. The curvature of the negative phototropism is the angle between the
elongation direction and the gravitation line, and the curvature of the gravitropism is the angle
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between the elongation direction and the horizontal line.
1.3
Observation of the microstructures of rice root
Different parts of rice root were transversely cut into fractions about 2 mm in length. These
fractions were firstly fixed with 2% glutaraldehyde, 1% paraformaldehyde, 0.05 mol L−1 sodium
cacodylate (pH 7.2) for 3 h. Secondly they were fixed with 1% osmic acid for another 3 h. Later
they were catena dehydrated with ethanol, displaced with epihydrin, and finally saturated and
embedded with low viscosity Spurr resins. Semi-thin slices of 1 µm were made and dyed with 1%
toluidine blue (TBO), and the microstructures of the bending parts were observed through an optical microscope.
2
Results and analyses
2.1
2.1.1
The negative phototropism of rice root
The negative phototropism of the seminal root.
Seminal roots were generated in 12
d at 2530. Their growth tendency of bending away from light was clearly observed when the
roots reached 0.5 cm in length. With proper light intensity, the curvature of negative phototropism
was usually between 25° and 40°, and a few could reach 45°. But the curvatures decreased gradually with the elongating of the roots, weakening of root growth and increasing of the root weight.
Adventitious roots of rice were generated from coleoptile in 34 d after sowing. Initially they
grew in all directions. However, they started exhibiting the characteristic of the negative phototropism soon after being irradiated (fig. 2-3).
The curvature under continuous irradiance was larger than that under interval irradiance (fig.
2-3 and 4), which suggested that the vertical downward growth of rice roots was only affected by
the gravity without light, and that bending growth was the result of the dual effects of the gravity
and the light. According to these characteristics the serrated-shape of rice roots could be induced
with intermittent irradiance (fig. 2-4).
2.1.2 The negative phototropism of the adventitious roots.
The adventitious roots from
tillering node or an elongation node are stronger than seminal roots and have stronger growing
potential. The curvature of them was usually 40°50°, and some even beyond 60° (fig. 2-2, 9
14). The curvature of adventitious root of a higher node was often larger than that of a lower node
(fig. 2-14), and the curvatures of the adventitious roots irradiated from the beginning were larger
than those irradiated after the initiation of adventitious roots in the darkness for a period.
The curvature of negative phototropism of rice root is the vector sum of negative phototropism and gravitropism[5,7]. The effect of gravity and the effect of light on root growth could be
distinguished from the curvature , that is, the curvature should be 45°if the effect of light was
equal to that of gravity. The curvature of seminal root was only 25°40° (less than 45°), which
indicated that the effect of gravity on the growth of seminal root was larger than that of light. But
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Fig. 2. Effect of light and gravity on the growth of rice root. 1, Adventitious roots grow in darkness, 0.7; 2, the bending
growth of adventitious roots under a continuous irradiance, 0.7; 3, seminal roots under a continuous irradiance for 3 d (irradiating from right to left. The light intensity of the near end to light source was about 100 µmol m−2 s−1, and that of the far end
was about 80 µmol m−2 s−1). The curvature of the near end was larger than that of the far end, 0.5; 4, effect of interval irradiance on root growth, 12 h irradiance, 12 h darkness. Arrows to the left stand for the irradiance, and arrows to the right stand
for the darkness, 1; 58, response of the roots and shoots to the gravity, 1.5: 5, turning 90° after the seed germinated for 30
h in darkness; 6, 2 h after having been horizontally placed; 7, 4 h after having been horizontally placed; 8, 6 h after having been
horizontally placed; 914, the effect of unilateral irradiance (from right to left) on the growing direction of adventitious roots,
×0.3: 9, adventitious roots having grown vertically in darkness; 10, 6 h of irradiance; 11, 12 h of irradiance; 12, 24 h of irradiance;
13, 36 h of irradiance; 14, 48 h of irradiance; 15, the longitudinal structure of the bending part: the left is the irradiated side, the
right is the shaded side, 60; 16, cells of the shaded side; 17, cells of the irradiated side, 300.
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the effect of gravity on adventitious root was less than that of light, for the curvature of adventitious root was larger than 45°. In addition, owing to the fact that the curvature of a higher node
was larger than that of a lower node, and even larger than that of the seminal root, it could also be
assumed that the effect of gravity on the growth of adventitious root decreased as nodes heightened.
2.2
Perception site of negative phototropism and structure changes of the root
2.2.1 Perception site of the negative phototropism.
Rice roots grew vertically downward in
darkness (fig. 2-1), and a negative phototropic bending growth of root tips could be observed after
unilateral irradiance for 1.5 h. Several treatments were carried out (table 1). When the root tip was
shaded/cut, or when the root cap was divested completely, the root would not show negative phototropism any more. All these results suggested that the light perception site was at the root cap
and the bending site was at the elongation zone about 2 mm from the root cap.
Table 1 Results of partial shading and divesting treatments to rice roots which were vertically growing in darkness
Treatment
Growth direction of root tip
1. Darkness (control)
keeping on vertical growth
2. Unilateral irradiated
negative phototropic bending
3. Shade the zone 2 mm above root tip, irradiate the root tip
negative phototropic bending
4. Irradiate the zone 2 mm above root tip, shade the root tip
keeping on vertical growth
5. Divest root cap completely, irradiate the root
6. Preserve the original cells of root-cap when divesting the
root cap, irradiate the root
7. Divest 0.5 mm of root tip, (namely divest the root cap
and differentiation zone), irradiate the root
8. Divest 2 mm of root tip (namely divest the root-cap differentiation and elongation zone), irradiate the root
still elongate, but no negative bending
the newly grew root exhibited negative phototropic bending
after 1 d of no negative phototropism
limit elongation, no negative phototropic bending
stop growing, no negative phototropic bending
In addition, when the vertical growing roots (seminal roots or adventitious roots) were laid
on water surface or in water to study the effect of gravity on root growth, the gravity perception
site was found to be root cap. When the root cap was divested, the root failed to show gravitropism. The bending site was also at the elongation zone about 2 mm from root cap (fig. 2-5 and 6).
An interesting thing was that the root-tip continued splitting and growing but failed to show
the characteristics of negative phototropism and gravitropism after root cap was divested completely. However, if the original cells of root cap were well protected when the root cap was divested, the roots would restore the negative phototropism and gravitropism after one day, for the
preserved root cap original cells split and grew into a new root cap. The root cap, the light perception site of the negative phototropism, was russet in the rice root of strong growth potential, and
this russet pigment was assumed to be a kind of photoreceptors. For study, the pigment was extracted from root caps with ethanol. There was absorption in the range of 340450 nm, near ultraviolet and blue light. The absorption peak was around 340350 nm.
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2.2.2 The structure of the bending part of rice root.
491
Fig. 2-1517 clearly shows the struc-
ture of bending part that manifested negative phototropism of rice root. There was no obvious
difference in the cell number between the internal side and external side of the part while there
were many differences in the length and the size of the cell. The length of 50 cells from the internal side of the bending part and another 50 from external side were randomly measured. The result
was that the average length of cells from the external side was 57.212.8 µm, while that of cells
from the internal side was 46.414.9 µm. This indicated that the bending growth did not result
from the uneven split of cells but from the uneven growth of cells, which was caused by the larger
elongation of the irradiated side than that of the shaded side. And it was the same to the response
of the gravitropism.
2.3
The effects of environmental factors on the negative phototropism
It has been reported previously[8] that light intensity had
some prominent effects on the growth and curvature of the roots (seminal roots or adventitious
roots). In other word, strong light inhibited the growth of roots and enlarged the curvature of roots.
Results of fig. 3 indicated that the curvature of root increased as the light intensity increased in the
2.3.1 The effect of light intensity.
range of 0100 µmolm−2s−1, and the increment of curvature was even larger at 020
µmolm−2s−1.
2.3.2 The effect of light quality.
When the rice roots (seminal roots or adventitious roots)
were irradiated with light of different quality but of the same intensity, we found that in the range
of 400720 nm, blue-violet light (400480 nm) was most effective on inducing the bending
growth, green light (520 nm) was also effective, while orange and red light (600720 nm) had no
effect (fig. 4). This indicated that it might be the blue/ UV-A photoreceptor instead of phytochrome that induced the negative phototropism of rice roots.
Fig. 3. The effect of light intensity on the negative phototropic curvature (average of 6 measured values).
2.3.3
The effect of temperature.
Fig. 4. The effect of light quality on the negative phototropic curvature (average of 5 measured values). The light
intensity is 20 µmolm−2s−1.
Table 2 clearly shows that 30 was the most optimal tem-
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perature for the growth and negative phototropic response of rice roots. At 20and 40, the
roots continued to grow and showed negative phototropism but the growth increment and the curvature decreased. Chilling injury happened at 10, which meant that the roots stopped growing
and failed to show negative phototropism.
Table 2 The effect of temperature on the growth and negative phototropism of rice roots
Growing rate/mmd−1
Temperature/
10
Curvature of negative phototropism/(°)
0.80.8 (6)
0.00.0 (6)
20
8.31.9 (6)
19.32.2 (6)
30
19.72.7 (6)
36.17.7 (6)
40
13.13.5 (6)
23.77.2 (6)
Measured 24 h after treatment. Averagestandard deviation (sample number).
2.3.4 Effects of plant growth regulators.
Among many plant growth regulators, the effect of
IAA was especially prominent. Its inhibition to the elongation, the negative phototropism and
gravitropism of the rice roots rose up as its concentration increased in the proper range of concentration (table 3).
The rice roots showed no negative phototropism when the concentration of IAA was in the
range of 10100 mg L−1. The interesting thing was they restored the negative phototropism
when transferred to pure water. The higher the IAA concentration, the longer the time they could
restore (table 4). This indicated that the negative phototropism was related to IAA. The endogenous IAA concentration in a root tip was probably below 0.1 mg L−1, for that the growth and the
negative phototropism of root was inhibited by IAA beyond 0.1 mg L−1 (table 3).
Gibberellin (GA) of 10 mg L−1 and 100 mg L−1 did not inhibit the negative phototropism,
but only promoted the elongation of the root, and decreased a little in the curvature (table 3).
In addition, the colchicine inhibited the development of canaliculus, the splitting and the
growing of the root tip cells. So the roots treated with colchicines stopped growing, showed no
negative phototropism and the root tips expanded like a ball.
Table 3 The effects of some plant growth regulators on the adventitious roots of rice
Types of hormone
IAA
Concentration/mgL−1
Pure water
Curvature of negative
phototropism/(°)
Curvature of
gravitropism/(°)
100
0.00.0 (10)
0.00.0 (10)
0.00.0 (10)
10
0.50.4 (10)
0.00.0 (10)
12.810.8 (10)
1
7.62.3 (10)
27.85.0 (10)
40.64.99 (10)
16.61.8 (10)
33.53.9 (10)
62.35.1 (10)
100
22.11.9 (8)
38.52.6 (8)
57.65.1 (8)
10
20.62.0 (8)
37.76.0 (8)
53.67.4 (8)
16.94.9 (10)
39.56.6 (10)
65.76.7 (10)
0.1
GA
Growth rate/mmd−1
Measured 24 h after treatment. Averagestandard deviation (sample number).
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Table 4 The effects of IAA treatments
Treatments
1. Continuously dipping in 100
mg/L IAA, under unilateral
irradiance
2. Transfer the roots from 10
mg/L IAA to pure water, under
unilateral irradiance
3. Continuous dipping in 10
mg/L IAA, under unilateral
irradiance
4. Transfer the roots from 10
mg/L IAA to pure water, under
unilateral irradiance
5. Pure water
Growth rate/mmd−1
Curvature of negative
phototropism/(°)
Curvature of
gravitropism/(°)
0.00.0 (10)
0.00.0 (10)
0.00.0 (10)
10.10.4(10)
39.17.4(10)
60.88.6(10)
0.50.4 (10)
0.00.0 (10)
12.810.8 (10)
15.43.9 (10)
38.56.4 (10)
54.93.8 (10)
16.94.9 (10)
39.56.6 (10)
65.76.7 (10)
Remark
Negative phototropic
bending demonstrated 6
h after treatment
Negative phototropic
bending exhibited 5 h
after treatment
Negative phototropic
bending exhibited 2 h
after treatment
Measured 24 h after treatment. Averagestandard deviation (sample number).
3
Discussion
The tropic movements of plant (including gravitropism, phototropism, thigmotropism,
chemotropism, etc.) are all growth movements, which result from the uneven growth of plant organs. The tropic movements will disappear when the organs are cut off or get old and stop growing. The negative phototropic bending occurs at the exuberantly growing root tip, which is due to
the larger elongation of cells on the irradiated side than that on the shaded side.
Three basic steps are included in the tropic movements of plant: () perception, the receptor
in plant receives unilateral stimuli from the environment; () transduction, the perception cell
transforms the environmental stimuli into physical or chemical signals; () motor response, the
growing organ grows unevenly and shows tropic movement after a signal perception[9]. The negative phototropic bending of rice root is supposed to include the above three basic steps.
The results of this experiment concluded that the root cap was the perception site, and that
blue/UV-A photoreceptor might be the photoreceptor in the root cap. The proofs of the former
conclusion were: () the negative phototropic bending could not be induced if the root cap was
shaded when the root was irradiated; () the root lost the characteristic of negative phototropism
if the root cap was divested; () the rice root could restore the characteristic of negative phototropism after the new root cap grew up, if the original cells of root cap were well protected when
root cap was divested. It was also pointed out by Fumio Takahashi that plant apex (stem tip or root
tip) was the part that would grow toward the most fitting direction after light signals perception[10].
The proofs of the latter one were: () the negative phototropic bending could be induced prominently by blue/UV light and green light, while red and far-red light had no such effect; () the
extractives from root cap absorbed near-violet and blue/UV light (320450 nm). Previous researches have proved that it was blue/UV light that induced the phototropic bending of plant stems,
and that the photoreceptor was blue/UV-A photoreceptor[11
ü13]
. The chromophore of the blue
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light/UV-A photoreceptor absorbs near-violet and blue/UV light, and some chromophores also
absorb green light[14].
Recently, Kiss et al. indicated that there were two photosensory systems that modulated
phototropic response in the roots of Arabidopsis mutant, which have weak gravitropism. One was
red-light regulating system with phytochrome as photoreceptor, which induced positive phototropism. The other was blue-light regulating system with blue-light photoreceptor, which induced
negative phototropism. The negative phototropism had much stronger effect than the positive
one[15]. However, in our experiments no positive phototropism was observed to be induced by
red/far-red light, probably because of lack of phytochrome in rice roots, or the phytochrome did
not operate much in the positive phototropism of the rice roots, and the gravitropism masked the
positive phototropism of the rice roots simultaneously. We plan to find some rice mutants that
have weak gravitropism for red/far-red light irradiating tests to identify whether there is a phytochrome regulated photosensory system in rice roots.
Root cap is the perception site of light stimulation. With the uneven growth of cells in meristem which was induced after receiving certain signals transferred from the root cap, they turned
into the cells in elongation zone which showed the negative phototropic bending. Although it is
still not sure through which signaling pathway does the photoreceptor transfer signals between a
root cap and a meristem, from the results of our experiment it could be supposed that this signal
might be IAA. The reasons are as follows: () part of the effect of light on a plant development is
due to hormone. The regulation of light to the elongation of plant stem needs IAA as a medium[16].
IAA is the only known hormone with polar transport[17], which can broadly regulate plant development[16
ü19]
. Apical dominance, cell elongation and differentiation, phototropism and negative
gravitropism of the stem, gravitropism of root all need IAA as a medium. () IAA with high concentration inhibits cell elongation, while IAA with low concentration promotes root elongation,
and our results also proved that the concentration of exogenous IAA could prominently affect the
negative phototropism of rice roots (tables 2 and 3).
Lino has confirmed that unilateral irradiance of blue light induced asymmetric distribution of
the IAA in coleoptile of maize (Zea mays), which led to the phototropic bending subsequently[20].
Light could also regulate plant growth by changing the level of endogenous hormone[21]. Yu et al.
found that the activity of IAA oxidase was increased in blue light-treated seedlings, and that the
decreasing of endogenous free IAA level was due to the augmental oxidation by increased IAA
oxidase activity[22]. So, we inferred there might be two ways how blue light induced negative
phototropism of rice roots: () blue light promoted the transportation of IAA to the shaded side;
() blue light promoted the oxygenolysis of IAA, which resulted in the decreasing of IAA of the
irradiated side and the increasing of IAA of the shaded side, and this uneven distribution of IAA
led to the negative phototropic response of rice root.
Young et al. reported in 1990 that the IAA was very important to gravitropism, which was
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495
first transported from the growing point to the root cap in the radical of maize. When the maize
coleoptile was laid horizontally, the IAA would be distributed asymmetrically in root cap under
the effect of the gravity. And IAA was transported from the root cap back to the growing part,
which led to the uneven growth and caused the response of the gravitropism[23
ü25]
.
Therefore, we inferred that the mechanism of the negative phototropism of rice roots was
similar. The IAA in root cap was redistributed under unilateral irradiance, and the IAA was transported from root cap to elongation zone, which resulted in the uneven growth and response of
negative phototropism. Further researches will be carried on to determine whether there are some
other causers.
The negative phototropism and gravitropism are parts of plant adaptabilities to the environment, which favor the roots to grow downwards into the soil and are also the physiology foundation of the standing of broadcasted rice seedlings. The shoots and roots are lying in the paddy field
after being broadcasted. Root tips of the levelly laid roots or the new adventitious roots grow
downwards into the soil rapidly under the dual effects of the gravity and the irradiance from the
sun. Meanwhile the negative gravitropism of the stem promoted rooting and standing of broadcasted rice seedlings.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.
30070454) and State Key Laboratory of Plant Physiology and Biochemistry (Grant No. 2002001).
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